Endothelial cells form a continuous surface that communicates with all parts of the body, yet they exhibit functional and structural specializations in various segments and locations of the vasculature. For instance, endothelial cells that line the capillaries of the brain are relatively thick, exhbit low permeability to macromolecular and high electrical resistance. Endothelial cells that line the capillaries of the intestine are thin and atenuated, have high permeability to macromolecules, and low electrical resistance. Even within a single organ endothelical cell specialization is evident. Arteriolar and true capillary endothelial cells lack histamine receptors while post-capillary venules possess these receptors. Most importantly, from the standpoint of inflammation, post capillary venules are the sites of leukocyte emigration. The research outlined in this proposal is aimed at studying the surface properties of endothelial cells and the role(s) of their surfaces in regulating leukocyte emigration. We describe a method for growing endothelial cells on amniotic membrane such that the endothelial cell monolayers exhibit silver stained intercellular junctions, are impermeable to macromolecular tracers, and show resistance to the passage of electrical current. We believe these cultures are excellent models of vessel walls. The studies proposed will use this "model" vessel wall to investigate the structure of the surfaces of endothelial cells derived from different anatomic locations, alterations in the surface properties of endothelial cells cultured on natural (amnion) and artificial (plastic) substrates, the structure of endothelial cell intercellular junctions, and the mechanisms by which leukocytes open these junctions and emigrate into the extravascular compartment.